Fix tox-dev#426 by writing directly to stdout buffer if possible to prevent str vs bytes issues. (tox-dev#739)

* Fix tox-dev#426 by writing directly to stdout buffer if possible to prevent str vs bytes issues. * Don't use assert in production code.

Author: fschulze

CONTRIBUTORS

@@ -5,6 +5,7 @@ Alexandre Conrad
 Allan Feldman
 Andrii Soldatenko
 Anthon van der Neuth
+Anthony Sottile
 Asmund Grammeltwedt
 Barry Warsaw
 Bartolome Sanchez Salado

changelog/426.bugfix.rst

@@ -0,0 +1 @@
+Write directly to stdout buffer if possible to prevent str vs bytes issues - by @asottile

tox/session.py

@@ -159,20 +159,25 @@ def popen(self, args, cwd=None, env=None, redirect=True, returnout=False, ignore
  self.report.logpopen(popen, env=env)
  try:
  if resultjson and not redirect:
- assert popen.stderr is None # prevent deadlock
+ if popen.stderr is not None:
+ # prevent deadlock
+ raise ValueError("stderr must not be piped here")
+ # we read binary from the process and must write using a
+ # binary stream
+ buf = getattr(sys.stdout, 'buffer', sys.stdout)
  out = None
  last_time = time.time()
  while 1:
  # we have to read one byte at a time, otherwise there
  # might be no output for a long time with slow tests
  data = fin.read(1)
  if data:
- sys.stdout.write(data)
+ buf.write(data)
  if b'\n' in data or (time.time() - last_time) > 1:
  # we flush on newlines or after 1 second to
  # provide quick enough feedback to the user
  # when printing a dot per test
- sys.stdout.flush()
+ buf.flush()
  last_time = time.time()
  elif popen.poll() is not None:
  if popen.stdout is not None: